Summary of site:
This road cutting shows the transition between the Mullaghmore Sandstone Formation and the overlying Benbulben Shale. Both are Carboniferous in age. It is the only exposure of this contact in the area and the top of the Mullaghmore Sandstone has a rich fauna of microscopic foraminifera that fixes its age.
About 1.5m of rock is exposed along a length of 60m in the wall of the cutting, overlain by 3m of glacial drift. The Mullaghmore Sandstone is initially a brown to grey, medium grained sandstone in beds 20-25cm thick, becoming progressively more lime rich and muddy upwards. It is topped by 25cm of shelly limestone with algal nodules (deposited by the microscopic plants). It is this limestone that has yielded 20 or so species of foraminifera. Only 60cm of Benbulben Shale can be seen in the cutting, mostly dark grey shale with silty sandstones.
The Mullaghmore Sandstone is a deltaic formation and this locality shows the final stages of the delta top before it was inundated by the deepening sea at the onset of Benbulben Shale times. The foraminifera fix the transition in the middle to late part of the Arundian stage, around 344 million years ago. Conditions at the time were tropical, as the area approached the equator in its northward drift.